1. Technical Field
The present disclosure relates in general to wireless communication devices and in particular to channel estimation in wireless communication devices.
2. Description of the Related Art
In the LTE specification, the transmitted signal is described by a resource grid, which has frequency on one axis and time on the other axis. Each element of the grid can be identified by subcarrier number in the frequency direction, and symbol number in the time direction. In a normal sub-frame, the reference signal is transmitted on a semi-regularly distributed subset of the grid. These reference signals are filtered to create channel estimates for the data elements of the resource grid. The reference signals are spread out in time and frequency, but only occupy some of the symbols in the grid. Furthermore, in the LTE standard, not all sub-frames are normal.
The number and position of symbols that contain reference signals varies. In particular, the number and position of symbols that contain reference signals depends on whether the received sub-frame is a multicast-broadcast single frequency network (MBSFN) sub-frame or a special time division duplex (TDD) sub-frame. In addition, the presence of measurement gaps, during which reference signals are not available, increases the difficulty in determining reliable channel estimates. Conventionally, various types of filters are applied to reference signals in order to obtain channel response estimates.
One method for filtering the reference signals is to first filter in the frequency direction to obtain initial channel estimates. Then, using the initial channel estimates, a second filtering operation is done in the time direction. However, in some cases, initial channel estimates are only available for the ends of the time direction filter with no initial channel estimates available for the middle. In other cases initial channel estimates are available only for the middle of the time direction filter. Another challenge involves a generation and selection of filter coefficients that account for channel conditions, such as Doppler rate. A further challenge involves generation and selection of coefficients that account for the latency requirement of the physical channel for which channel estimates are being calculated.